This invention relates to a facemask, and in particular to a facemask for use in a closed circuit breathing system.
With a facemask for use in an open circuit breathing system, wherein a compressed air bottle is used as an air source and exhaled gas from the user is released from the facemask, it is not so difficult to prevent fogging of the eye piece of the facemask because the breathing air from the compressed air bottle may be directed across the eye piece to keep it clear.
However, in a facemask for use in a closed circuit breathing system, often adopted for a portable breathing apparatus, and wherein oxygen is supplied from a chemical oxygen generating apparatus or an oxygen cylinder, carbon dioxide gas is merely removed from the exhaled gas without venting the exhaled gas from the system. The carbon dioxide free exhaled gas is then mixed with fresh oxygen from the oxygen cylinder and is recirculated as breathing gas. Such a closed circuit facemask involves a large number of difficult technical problems. Above all, the most important problems are how to maintain the facemask airtight and to prevent fogging of the eye piece. That is, since a closed circuit facemask is intended to utilize oxygen generated from an oxygen supply source having a limited capacity as efficiently as possible, attention must be paid to the enhancement of the airtightness between the facemask and the face of the user so that no oxygen or breathing gas escapes from this area. However, with these completely closed facemasks, moisture gradually accumulates in the system due to the moisture contained in the exhaled gas and facial perspiration and condenses on the eye piece of the facemask, thereby fogging the eye piece.
Further, conventional facemasks, because of their large or protruding eye piece and/or full face seals, from an obstruction to the wearing of a helmet, such as a fireman's helmet. The facemask seal is often dislodged by the helmet, or the helmet does not fit properly over the facemask and its attachment, i.e. straps, buckles, etc.
Furthermore, most facemasks accommodate head size variations by employing straps and buckles across the back of the head and neck of the user which require adjustment. These straps and buckles are complicated to adjust. In many conventional applications, donning the facemask must be delayed until the breathing apparatus is actually used due to semi-permanent attachment of the breathing hoses to the facemask.
Also voice communication is generally hampered during use of the facemask since the mouth and nose, and sometimes the ears, are covered.